Combination chemotherapy regimens for advanced breast cancer usually result in objective responses as first line treatment. However, complete remissions occur in fewer than 20% of patients, and median survival is limited. Therapeutic strategies that would elicit a clinical response in refractory advanced stage breast cancer are needed. ONTAK, a cytotoxic fusion protein composed of diptheria toxin (DT) fragments and amino acid sequences of IL-2, is designed to direct the cytocidal action of DT to malignant cells that express the high affinity form of the IL-2 receptor (CD25) while minimizing damage to normal cells that do not express the receptor. Clinical studies have shown ONTAK to be a safe and effective agent in the treatment of cutaneous T cell lymphomas (CTCL) that express CD25. Its putative mechanism of action in CTCL is to bind to the interleukin-2 receptor (IL-2R) and induce uptake of the drug with subsequent inhibition of protein synthesis by the diptheria toxin. Studies have clearly shown the capacity of the drug to block protein synthesis and induce cell death. Breast cancer, like CTCL, has been shown to express CD25, and this increased expression of CD25 is associated with the malignant potential of the tumor. Thus, we propose that, similar to its action in CTCL, ONTAK may have direct anti-tumor activity in breast cancers that overexpress IL-2R. Accumulating evidence suggests that a population of CD4+ T cells that constitutively express the IL-2Ra chain may function as "professional" suppressor or T regulatory cells (Tregs). Tregs are potent suppressors of CD4+ and CD8+ T cells and have been shown to down-regulate immune responses to self antigens, such as tumor antigens. Additionally, Tregs have been shown to be increased in the peripheral blood of patients with breast cancer. We hypothesize that depletion of Tregs by ONTAK may result in generation of functional immune effector cells and enhancement of endogenous tumor-specific immunity. This proposal is a phase I-II study of ONTAK administered to patients with advanced refractory breast cancer. The specific aims of this study are to (i) evaluate the safety of ONTAK administration in patients with advanced refractory breast cancer, (2) determine whether administration of ONTAK in patients with advanced stage breast cancer will decrease circulating T regulatory cells and augment endogenous immunity, and (3) evaluate an association between IL-2R expression in breast cancer and any clinical response induced by ONTAK administration. [unreadable] [unreadable] [unreadable]